The invention pertains to a method for operating a driving system with an internal combustion engine and an exhaust gas purifying device, through which the exhaust gas of the internal combustion engine flows to be purified, wherein the exhaust gas purifying device comprises at least one catalyst element for the catalytic conversion of nitrogen monoxide to nitrogen dioxide at a determined conversion rate. The invention also pertains to a driving system.
The driving system serves, for example, to drive a motor vehicle or a watercraft. For this purpose, a torque is provided by the internal combustion engine, which is configured as, for example, a diesel engine. The exhaust gas produced by the internal combustion engine is sent to the exhaust gas purifying device, where it is least partially freed of pollutants. It is frequently advantageous for the exhaust gas flowing through the exhaust gas purifying device to comprise a certain percentage of nitrogen dioxide, so that it can be used, for example, to regenerate a particle filter and/or to react with nitrogen monoxide in a catalyst.
Only a small quantity of nitrogen dioxide or possibly none at all, however, is present in the exhaust gas immediately downstream from the internal combustion engine. In contrast, the amount of the nitrogen monoxide in the exhaust gas is comparatively large. It is for this reason that the at least one catalyst element is provided in the exhaust gas purifying device. This catalyst element serves to convert nitrogen monoxide catalytically to nitrogen dioxide, wherein this takes place at the determined conversion rate. Downstream from the catalyst element, i.e., downstream relative to the main flow direction of the exhaust gas, a higher percentage of nitrogen dioxide and/or a larger quantity of it is therefore present in the exhaust gas of the internal combustion engine than is present upstream of the catalyst element. The catalyst element comprises at least one catalytically active component or catalyst material such as platinum or a platinum alloy. As the service life of the catalyst element increases, however, the catalytic effect of this catalyst material and therefore of the catalyst element decreases, in particular as a result of thermal aging and/or the chemical positioning unavoidably caused by certain components of the lubricants and/or fuels used to operate the internal combustion engine. The quantity of nitrogen dioxide produced by catalytic conversion decreases to a corresponding extent.
For this reason, the catalyst element is usually configured in such a way that it allows a conversion rate to be achieved which is greater than or equal to a certain minimum conversion rate over its entire planned service life. This means, however, that, at the beginning of the service life of the catalyst element and in particular immediately upon the startup of the driving system, the conversion rate is higher than is necessary to produce the desired quantity of nitrogen dioxide. To a corresponding extent, the quantity of nitrogen dioxide present in the exhaust gas downstream of the catalyst element is too large; under certain circumstances, no use can be made of this surplus downstream from the catalyst element, and to this extent it is discharged to the external environment of the driving system. The result is that the nitrogen dioxide emissions of the driving system are higher than need be.